You can almost always hear a "gurgle" start or change sound at the fill
deck fitting when the tank is getting full. At that point go slow. I've had the whistler an it helps. but doesn't stop whistling until it fills with
fuel. That may be too late unless you are filling quite slow. I guess the gurgle and whistle are about equal use.
Although maybe it's possible to install the whistler down near the tank top (?) instead of further up the vent line....